In recent years, PLC (Power Line Communication) data communication system in which a power line for commercial power supply is used to bidirectionally connect between electrical appliances has attracted attention at home or in office.
In this system, a power-line communication apparatus is set at an arbitrary position in indoor wiring, and data is transmitted via a coupling circuit provided in this communication apparatus, thereby performing data communication between communication apparatuses and between a communication apparatus and an external communication apparatus. An outdoor power line is connected to the indoor wiring via a filter that passes only an alternating current of, for example, several hundreds of Hz or less. A parent MODEM is connected to the indoor wiring, and the parent MODEM has a router function and is connected to the Internet (server) via an optical terminal device, an optical fiber line, and others.
The indoor wiring is branched at an arbitrary position and connected to arbitrary loads such as a personal computer, a television, a refrigerator, and other electrical appliances via sockets and switches. Some of these electrical appliances have a child MODEM and perform data communication with the parent MODEM or other child MODEM via the indoor wiring by using, for example, a predetermined band in a shortwave band.
The indoor wiring is branched at a socket, and the line length and the terminal impedance thereof are changed when a new load is connected to or removed from the socket or when an arbitrary load is turned ON/OFF. Therefore, signal reflection and resonance occur in accordance with the line length to the branching point or the terminal, the terminal impedance, and others.
For this reason, in the indoor wiring, attenuation sometimes occurs over a wide band in the use band or no signal is transmitted at a specific frequency (spectrum-null point) in some cases. Moreover, the frequency spectrum of the transmission path is frequently changed depending on the load state and other factors.
Furthermore, as another problem in the indoor wiring, not only noise generated from a load but also an interference signal and noise induced by the resonance of the indoor wiring serving as an antenna with an electromagnetic wave in the air are added at a level equivalent to that of the signal power.
As the above-described PLC data communication system, the system performing the communication by OFDM (Orthogonal Frequency Division Multiplexing) has been proposed (refer to Japanese Patent Application Laid-Open Publication No. 2003-188781).
In this OFDM transmission scheme, however, if roll-off of a transmission pulse is decreased in order to increase frequency use efficiency, the length of a guard interval signal is increased, and the transmission efficiency is disadvantageously decreased. Further, since the OFDM transmission scheme is a block transmission scheme, blocks have to be reassembled at the time of fluctuations in the transmission path, and quick response is difficult.
Furthermore, as a generally known technology, when the characteristic of the transmission channel becomes deteriorated, a fallback mode is adopted to decrease a data transmission rate (refer to Japanese Patent Application Laid-Open Publication No. 2000-22771).
In this technology, however, the number of signal points forming one symbol (the number of modulation multi-values in digital modulation) is decreased or the same symbol is repeatedly transmitted. Therefore, when the transmission quality is decreased, fallback is performed without considering the spectrum amplitude of the transmission channel.